1. Field of the Invention
This invention relates to a method of hardening gelatin using an improved hardening agent and particularly to a method of hardening gelatin for silver halide photographic light-sensitive materials.
2. Description of the Prior Art
Gelatin has been used as a binder for many kinds of photographic light-sensitive materials. For example, gelatin has been used as a main component for silver halide light-sensitive emulsion layers, emulsion protective layers, filter layers, intermediate layers, antihalation layers, backing layers, subbing layers of film bases or baryta layers, and so forth.
These light-sensitive materials containing the gelatin are processed with various kinds of aqueous solutions, each with a different pH or with a different temperature. The properties of layers containing a gelatin which is not processed with a hardening agent depend mainly upon the properties of the gelatin used, and such layers typically have poor water resistance and low mechanical strength due to excessive swelling in the aqueous solutions. In extreme cases, the gelatin layers sometimes dissolve off into the solutions when aqueous solutions at a temperature higher than about 30.degree. C. or highly alkaline aqueous solutions are used. This characteristic of dissolving under such conditions is a fatal defect for layers in photographic light-sensitive materials.
However, a number of compounds are known to be effective for hardening gelatin to improve the water resistant properties, heat resistant properties and scratch resistant properties of gelatin layers.
These compounds are known as "hardening agents" in the production of photographic light-sensitive materials. For example, known gelatin hardening agents include aldehyde compounds such as formaldehyde or glutaraldehyde, reactive halogen containing compounds as described in U.S. Pat. No. 3,288,775, et al., compounds having ethylenically unsaturated reactive bonds as described in U.S. Pat. No. 3,635,718, et al., aziridine compounds as described in U.S. Pat. No. 3,017,280, epoxy compounds as described in U.S. Pat. No. 3,091,537, halocarboxyaldehydes such as mucochloric acid, dioxanes such as dihydroxydioxane or dichlorodioxane, and inorganic hardening agents, such as chromium alum or zirconium sulfate, and so forth.
However, all of these known compounds have some disadvantages. Particularly, some have an insufficient hardening function when used for photographic light-sensitive materials, some cause undesirable changes to occur in image quality over a period of time, because of a hardening function called "post-hardening" which occurs due to a slow hardening reaction with gelatin, some compounds adversely influence the properties of the photographic light-sensitive materials (particularly increasing fogging and reducing sensitivity, etc.), some lose their hardening ability when certain photographic additives are present at the same time, or interfere with the functions of these other photographic additives (for example, couplers for color light-sensitive materials), some are difficult to synthesize in large quantities, and some have poor storability because they are unstable.
It has previously been proposed to use a post-hardening free hardener (that is, a hardener that can be used without the occurrence of the post-hardening phenomenon; also, as used herein the term "hardener" refers to a composition comprising a "hardening agent" compound, either alone or with other substances) comprising a compound having at least two N-acyloxysuccinimide groups in the molecule (Japanese Patent Publication No. 22089/78) or a compound having an N-sulfonyloxysuccinimide group (German Patent Application (OLS) No. 2,704,276). Such a hardener causes very rapid hardening of gelatin and substantially eliminates the possibility of post-hardening. The hardener is also free from such undesired effects as increased fogging and reduced sensitivity (caused by either the hardener itself or by a reaction by product thereof). Furthermore, it does not interact with other photographic additives, such as a coupler for color sensitive materials, to either interfere with the function of these additives or lose its hardening ability. Unfortunately, most of these otherwise excellent hardeners are low in water solubility, and require a special organic solvent for incorporation into a photographic emulsion or a photographic emulsion layer.